Go to your Quests list, and see first if you have had the required amount of help to complete the quest ("Share the Wealth").

Then, go to your maps page, on the bottom left click "show active quests", and place your cursor over the "Share the Wealth" button (don't click on it). The map will move, and the zone in which the treasure chest is located will light up.

If the button is red, that means your quest is in one of the hidden zones (Wonderland, etc.), and you can click to those hidden areas until the button turns green, then you can find the chest using the last paragraph.

You need to go to ANY SQUARE, in ANY side map that you may have visited. You also need to check out areas like the warriors tomb, the crypt (in Haven), the caves, the pyramids, in short, any area you may have been in. Go into the areas, and open the map and once the quest turns green, you'll see the treasure.

The Haunted Castle IS one of the 'side maps' that Monica mentioned. There are 9 of them, and there are other areas on the main map that are actually unmapped. But it's good that you are squared away.
The Developers are coming out soon with something else that will be even better that the 'show active quests' feature.